1. Field of the Invention
This invention is in the field of computer-generated raster graphics and more particularly relates to apparatus in which the memory requirements are reduced without reducing the number of behaviors, or colors, that can be displayed by the cathode ray tube subsystem (CRT) of the system, and in which either the background or the foreground behaviors can be independently changed.
2. Description of the Prior Art
Raster scan CRT displays form a principal communication link between computer users and their hardware/software systems. The basic display device for computer-generated raster graphics is the CRT monitor, which is closely related to the standard television receiver. In order for the full potential of raster graphics to be achieved, such displays require support systems which include large-scale random access memories and digital computational capabilities. As the result of recent developments of large-scale integrated circuits, the price of digital memories has been significantly reduced, and microcomputers are now available with the capability of controlling such displays at affordable prices. As a result, there has been a surge of development in raster graphics. Typically, each PIXEL in a rectangular array of a picture element (PIXEL) of a CRT is assigned a unique address, comprising the X and Y coordinates of each PIXEL in the array. Information to control the display is stored in a random-access memory (RAM) at locations having addresses corresponding to those assigned to the PIXELS. The source of PIXEL control data stored in the RAM is typically a microcomputer located in a graphic controller which will write into the addressable memory locations of the RAM the information necessary to determine the type of display. This frequently includes an address in a color look-up memory, at which location there is stored binary color control signals to control the intensity of the color of each pixel of an array. The horizontal and vertical sweeps of the raster scan are digitized to produce addresses of the PIXELS, which addresses are applied to the memory in which the controller has previously written information determinative of the display; i.e., the color and intensity of the addressed PIXEL as it is scanned. As stated above, this information can be an address in a color look-up memory. The digital color control signals are read out of the addressed locations in the color look-up memory. The digital color control signals are converted to analog signals and applied to the three color guns of the typical CRT to control the intensity and color of each PIXEL as it is scanned.
A well-known technique for controlling the displays of such a system is to have a PIXEL memory containing a PIXEL image of the display in each addressable location of which is stored a background/foreground control bit and a corresponding behavior memory that describes the color, or behavior, of each PIXEL. The size of the memory required is equal to the number of PIXELS times the sum of the background/foreground bits plus the number of behavior bits, typically four per PIXEL.
There is a need in color graphic systems to reduce the memory requirements of such systems without a commensurate degradation in the versatility of colors displayed by each PIXEL.